Module 5 : Electrochemistry
Lecture 24 : Applications of Electrode Potentials.
  24.2

Determination of Solubility Products of Sparingly Soluble Salts

The solubility product Ksp (ABn An+ + n B - ; Ksp is defined as Ksp= [A n+ ] [B - ] n ) is an equilibrium constant and its value can be predicted from a knowledge of the standard electrode potentials. The electrodes are chosen in such a way , that the overall cell reaction is the solubility product equilibrium. A suitable galvanic cell is thus set up and its emf is determined experimentally. Even if measurements can not be made, it is possible to arrive at the approximate values of Ksp from appropriate half cell reaction potentials as shown below.

 
We will now illustrate the calculation of Ksp for Hg2Cl2 (s) at 298 K from data on electrode potentials.
Hg2Cl2 (s) Hg22+ + 2Cl - ; (24.3)

Ksp = [ Hg22+ ] [ Cl - ] 2

(24.4)
 

We need to look for a reaction in which Hg2Cl2 takes part and another reaction in which Hg22+ and Cl - take part. These reactions are

Hg2Cl2 + 2e - 2Hg + 2Cl -; E o = + 0.268 V

Hg22+ + 2e - 2Hg ; Eo = 0.789 V
Reaction ( 1 ) - Reaction ( 2 ) gives

Hg2Cl2 Hg22+ + 2Cl - ; Eo = 0.268 - 0.789 = - 0.521 V

Go = - RT /n Keq = - nFEo , Keq = [Hg22+ ] [Cl - ] 2 / [Hg2Cl2] (24.5)
= Ksp [ Hg2 Cl2 ] (24.6)
= Ksp as the activity of the solid [Hg2Cl2] is taken to be 1. We should be using activities all through. Since the concentrations of most of the ions of these sparingly soluble salts are less than 10 -4 M, activities have been replaced by concentrations.
 

Ksp = exp [nFEo / RT] or 10 [ nEo / 0.051T] = 1.355 x 10 -18

(24.7)
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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